Thar Desert

Also called: Great Indian Desert, Marusthali · Topic: Structure and Physiography of India: Himalayas, Plains, Peninsula, Coasts and Islands · NCERT: Class 6, Ch 3 "Landforms and Life"; Class 7, Ch 1 "Geographical Diversity of India"; Class 7, Ch 3 "Climates of India"; Class 11, Ch 1 "India - Location"; Class 11, Ch 2 "Structure and Physiography"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"

Meaning

The Thar Desert, also called the Great Indian Desert or Marusthali, is a dry, sandy land north-west of the Aravalli Range. It gets under 150 mm of rain a year and is covered with longitudinal dunes and barchans. It sits on old Peninsular rock, and the region lay under the sea in the Mesozoic era.

It matters because it shows how wind and heat shape land. It also shows how people live with very little water, and how canals and overgrazing can change a landscape.

Explanation

Structure: old rock below, wind-made land above

• Base rock: the desert sits on the Peninsular Block (the old, hard and stable Gondwana landmass).

• So in structure, the desert is a westward continuation of the Peninsula.

• Once a sea: in the Mesozoic era (about 252–66 million years ago), this region lay under the sea.

• Akal (Aakal) Wood Fossil Park near Jaisalmer has fossil tree trunks about 180 million years old (Lower Jurassic).

• Brahmsar near Jaisalmer has marine deposits (rocks laid down on the sea floor, with remains of sea life).

• How petrified wood (wood turned into stone) forms:

• Trees growing near the sea were buried quickly under sediment.
• Water carrying silica soaked into the wood.

• Over time the silica replaced the wood, leaving a stone copy with its rings still visible.

• Lesson: the land has changed from sea to coastal forest to desert. A desert is not permanent.

How the surface is shaped: heat and wind

• Physical (mechanical) weathering breaks rock without changing its chemistry.
• Hot days make rock expand, and cold nights make it shrink.
• This repeated stress cracks the rock, and its outer layers peel off.

• The loose bits become sand that the wind can carry.

• Wind then builds landforms:

• Longitudinal dunes: long, narrow sand ridges parallel to the main wind. They form where the wind blows steadily from one direction and there is a moderate amount of sand.
• Barchans: crescent-shaped dunes with horns pointing downwind. The gentle slope faces the wind and the steep slip face is on the sheltered side. They form where sand is limited and the wind blows mostly from one direction.
• Dunes can be up to 150 m high (NCERT Class 7).
• Mushroom rocks (pedestal rocks): sand carried by wind close to the ground cuts the base of a rock like sandpaper. The top is above the sand's reach, so the rock ends up with a thin base and a wide top.

• Shifting dunes: dunes that move as the wind pushes sand forward. They can bury fields, roads and villages.

• Oases: green spots where groundwater reaches the surface. In the Thar they are mostly in the south.

Feature	Longitudinal dune	Barchan
Shape	Long, straight ridge	Crescent (half-moon)
Direction compared with wind	Parallel to the wind	Horns point downwind
Sand supply	Moderate	Limited
Movement	Grows along its length	Moves forward as a whole (shifting dune)

Climate and vegetation

• Rainfall: under 150 mm a year, so the climate is arid (very dry).
• High diurnal range of temperature (a big gap between day and night temperatures):
• Dry air and bare sand let the ground heat up fast by day.
• With no clouds to trap the heat, the ground cools fast at night.

• So days are very hot and nights are cold.

• Vegetation: low and sparse. It is mostly thorny bushes and hardy grasses that survive drought.

Drainage: rivers that never reach the sea

• Slope: the north slopes towards Sindh (Pakistan), and the south slopes towards the Rann of Kachchh.
• Ephemeral rivers: these flow only after rain and stay dry for the rest of the year.
• Inland drainage: streams end in closed basins inside the desert instead of reaching the sea.
• Playas are shallow, flat lake beds. They fill after rain and dry in summer, leaving salt behind.

• Their brackish water (slightly salty water) is used to make salt.

• The Luni is the main river of the region:

• It rises near Pushkar in the Aravallis (Ajmer district) as two streams, the Saraswati and the Sabarmati, which join at Govindgarh.
• It flows west, then turns south-west.
• It ends in the Rann of Kachchh, not the sea.
• Its upper course is fresh, and its lower course is saline (salty).

Where it occurs

• In India: mostly western Rajasthan, plus parts of Gujarat, Punjab and Haryana (NCERT Class 7).
• Outside India: the desert continues west into Sindh (Pakistan).
• Boundaries:
• East: the Aravallis, which run south-west to north-east across Rajasthan.
• North: the plains of Punjab and Haryana.
• South: the Rann of Kachchh (Gujarat).

• West: the Indus plain of Pakistan.

• Key places:

• Jaisalmer, the "Golden City", named after its yellow sandstone. Jaisalmer Fort is a UNESCO World Heritage Site as part of the "Hill Forts of Rajasthan".
• Akal Wood Fossil Park and Brahmsar, both near Jaisalmer.
• Pushkar (Ajmer district), the source of the Luni and the site of a famous cattle and camel fair.
• Sambhar Lake (Rajasthan), India's best-known inland salt lake. It is fed by four streams, and its water level changes with the seasons [2]. It became a Ramsar site on 23 March 1990, covering 24,000 ha [2]. It supports many wintering waterbirds, including large numbers of flamingos [2].
• Indira Gandhi Canal (Rajasthan Canal): it starts at Harike Barrage in Punjab, where the Sutlej and Beas meet. It irrigates the north-western Thar (Sri Ganganagar, Bikaner, Jaisalmer and Barmer area).

Don't confuse with

• Rann of Kachchh: a seasonal salt marsh in Kachchh district, Gujarat, that floods in the monsoon and dries into a white salt flat. It lies south of the Thar, is not a sandy dune desert, and is where the Luni ends.
• Barchan vs longitudinal dune: a barchan is a crescent with horns pointing downwind (limited sand). A longitudinal dune is a long ridge parallel to the wind (moderate sand).
• Mushroom rock vs water-made landforms: a mushroom rock forms by wind abrasion (sand-laden wind cutting the base of a rock), not by running water.
• Banni grassland: this is a tropical grassland on the southern edge of the Great Rann in Gujarat (Maldhari herders, Banni buffalo). It is not part of the Thar in Rajasthan.

Prelims Hooks

• Marusthali = Great Indian (Thar) Desert. It lies north-west of the Aravallis, gets under 150 mm of rain a year and sits on Peninsular bedrock.
• Akal Wood Fossil Park is in Jaisalmer, Rajasthan. Its fossil trunks are about 180 million years old (Jurassic). Together with the marine deposits at Brahmsar, they show the area was under the sea in the Mesozoic.
• Luni: rises near Pushkar (Aravallis), ends in the Rann of Kachchh (not the sea), and its lower course is saline.
• Trap: the Thar has inland drainage. Most streams are ephemeral and end in playas, which are used for salt making.
• Sambhar Lake (Rajasthan) is an inland salt lake and has been a Ramsar site since 1990 (24,000 ha) [2].
• Order from north to south: Thar (Rajasthan) → Great Rann → Banni / Kachchh mainland, with the Little Rann to the south-east.

Mains Points

• A landscape that keeps changing (GS-I):
• The region went from Mesozoic sea (Akal, Brahmsar) to Jurassic coastal forest to today's wind-shaped desert.
• This shows how changes in climate and sea level remake landforms.

• Today, physical weathering and wind shape the surface, leaving dunes, mushroom rocks and playas.

• Living with little water (GS-I society and geography):

• People store rainwater in a taanka or kund (an underground tank that collects rain from rooftops or courtyards for drinking).

• They scrub pots with sand instead of water, and they depend on oases and herding.

• Environmental problems and responses (GS-III):

• Desertification (good land turning into desert): overgrazing and cutting of bushes leave the soil bare, and wind blows that sand onto farmland.
• Indira Gandhi Canal: it turned desert into fields of wheat, cotton and groundnut. But it also caused waterlogging (water stays in the soil because it cannot drain) and soil salinity (salt rises to the surface as water evaporates).
• Salt-lake wetlands like Sambhar are under pressure [2].
• Responses: dune stabilisation (planting grasses and shrubs on dunes to stop them moving) and shelterbelts (rows of trees that slow the wind).

Related concepts

• Akal Wood Fossil Park
• Rann of Kachchh
• Banni grassland

Read more

• The Indian (Thar) Desert and the Rann of Kachchh